An example of a secondary-side power receiving circuit of conventional noncontact power supplying equipment is disclosed in, for example, Japanese Patent Laid-Open No. 11-178104.
In the secondary-side power receiving circuit of the noncontact power supplying equipment, a capacitor forming, with a pickup coil, a resonance circuit resonating at the frequency of an inductive path is connected in parallel with the pickup coil, a rectifying/smoothing circuit is connected to the capacitor, a constant voltage control circuit for controlling an output voltage to a reference voltage is connected to the rectifying/smoothing circuit, and an inverter and a control power supply are connected to the constant voltage control circuit as an example of a load. An electric motor including a speed reducer is connected to the inverter.
The constant voltage control circuit is made up of a coil for limiting a current, a voltage generator for generating the reference voltage, a comparator for comparing the output voltage and the reference voltage, an output adjusting transistor including a FET which is connected between the output ends of the rectifying/smoothing circuit and is turned on by the comparator when the output voltage exceeds the reference voltage, and a diode and a capacitor which form a filter. The control power supply supplies control power to the voltage generator and the comparator.
With the configuration of the constant voltage control circuit, when a load decreases due to the stop and so on of the electric motor, the output voltage increases. When the output voltage exceeds the reference voltage, the output adjusting transistor is turned on by the comparator, and the output voltage is reduced and kept at the reference voltage.
However, according to the conventional configuration, when the load decreases due to the stop and so on of the electric motor, the output adjusting transistor is turned on and thus current passes through the output adjusting transistor even though power supply to the load is unnecessary, resulting in a loss.